Although reference will always be made to a capacitive fill-level sensor in what follows, the invention can basically be applied to any type of measuring device in which the sensor and the evaluating unit have a defined spatial distance from each other. Thus, the device in accordance with the invention, or the method in accordance with the invention, can also be employed in connection with a pressure sensor.
A capacitive fill-level probe is known from DE 195 36 199 C2, which is mounted at the level of the fill level to be monitored. Such probes are also called threshold level detectors and are mounted in the form of overflow protection devices in containers or, upstream of pumps, as protection devices against running empty. If the probe is covered by the filler material to be respectively detected, it has a larger capacitance than in the uncovered state. The measured capacitance value is compared with a threshold value by means of a capacitance measurement circuit and a comparator; from the result it is possible to determine whether the fill level to be monitored has been reached, or has not yet been reached. The setting of the threshold value, or of the switch point, is of course extremely critical in this connection. Therefore the solution disclosed in DE 195 36 199 has as its aim to propose an automatic method for optimizing the setting of the switch point.
The rope probe described in EP 0 857 954 is employed when the measurement using a fill-level sensor or pressure sensor is to be performed at a location which is not easily accessible from the outside. An example of this is the placement of a probe at a defined height inside a tank or a reservoir. The rope is used for fixing the probe in place. The electrical supply and unidirectional transmission of measuring signals between the probe and the evaluating unit integrated in a housing takes place at the same time.
The solution described in EP 0 857 954 A1 describes a device for fastening the rope to the probe in which the device withstands all process-related stresses, in particular large tensional forces. However, the laid-open publication does not contain any clue regarding a bidirectional data exchange between the probe and the remotely located evaluating unit.
Moreover, the adjustment of the sensor for correct setting of the switch point is of quite considerable importance for the dependable and correct operation of the sensor during the process.
Besides, tolerances in the electronic and mechanical components are compensated by the adjustment. Since sensors are customarily encapsulated following mounting, an adjustment, for example by means of the torsion of a potentiometer or the installation of an additional resistor, is no longer possible. Therefore the sensor must be laid out in such a way that it can be adjusted from the outside.